Wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines that are designed to provide electrical power to a utility grid can have large rotors (e.g., 30 or more meters in length). In addition, wind turbines are typically mounted on towers that are at least 60 meters in height. Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
Wind turbine blades have continually increased in size in order to increase energy capture. However, as blades have increased in size, it has become increasingly more difficult to control optimum energy capture. The blade loading is dependent on the wind speed, tip speed ratio (TSR) and/or pitch setting of the blade. TSR, as is understood by those of ordinary skill in the art, is the ratio of the rotational velocity of the blade tip to the actual wind speed. Generally, it is important to optimize the operation of the wind turbine, including blade energy capture, to reduce the cost of the energy produced. Pitch setting of the blades (i.e., the angle of attack of the airfoil shaped blade), provides one of the parameters utilized in wind turbine control. Typically, controllers are configured to adjust the rotational speed of the hub around which the blades rotate, i.e., the rotational speed, by adjusting the blade pitch in a manner that provides increased or decreased energy transfer from the wind, which accordingly is expected to adjust the rotor speed.
The amount of energy that can be extracted from wind is based on a number of factors including the swept area covered by the rotating blades. This, of course, is directly dependent on the length of the blades so that increases in extracted energy are dependent, for individual turbines, at least in part, on providing longer blades.
One issue arising from continually increasing turbine size, however, is the need to increase the torque handling capacity of the gearboxes. It would be advantages, however, to keep the generator torque low so that gearbox torque ratings, and consequently size and cost, may also be kept low.
Therefore, what is needed is a method for operating a wind turbine to permit operation in a high power coefficient (Cp) operating region for longer periods to permit control of the generator torque limit.